Feedback Voltage Divider Calculator and Formula

A feedback voltage divider calculator finds the fraction of an output voltage returned to a circuit’s feedback input through two resistors. Enter the upper and lower resistor values and the output voltage; it calculates the feedback voltage and divider ratio, which are useful when checking a voltage-regulator or amplifier feedback network.

Formula

For a simple two-resistor divider, the feedback voltage is:

Vf = Vout × R2 / (R1 + R2)

• Vf is the voltage at the divider junction, in volts (V).
• Vout is the voltage applied to the top of the divider, in volts (V).
• R1 is the upper resistor, from Vout to the junction, in ohms (Ω).
• R2 is the lower resistor, from the junction to ground, in ohms (Ω).

The divider ratio is β = R2 / (R1 + R2), so the same formula can be written as Vf = β × Vout. Both resistors must use the same resistance unit; ohms, kilohms, and megohms are interchangeable after conversion because only their ratio matters.

Worked Example

Using the calculator’s default values, R1 = 10 Ω, R2 = 10 Ω, and Vout = 5 V:

1. Add the resistors: R1 + R2 = 10 Ω + 10 Ω = 20 Ω.
2. Find the fraction across R2: R2 / (R1 + R2) = 10 / 20 = 0.5.
3. Multiply by output voltage: Vf = 5 V × 0.5 = 2.500 V.
4. The feedback ratio is β = 0.5, or 0.5000 when displayed to four decimal places.

How a Feedback Divider Works

A voltage divider consists of two resistors in series across a voltage source. The feedback node is their shared junction. With an ideal, unloaded divider, the same current flows through both resistors, and the junction voltage is the share of the total voltage dropped across R2. Increasing R2 relative to R1 raises the feedback fraction; increasing R1 relative to R2 lowers it.

In a closed-loop circuit, a controller or amplifier compares this returned voltage with a reference or input. It adjusts its output in response to the difference. For example, a common adjustable regulator arrangement targets a specified feedback-pin voltage. If that target is known, the divider can be selected so the desired output produces the target at the junction. The calculator here works in the forward direction: given output voltage and resistor values, it predicts the divider’s feedback voltage. It does not identify a device’s required target or guarantee that a circuit will regulate correctly.

Choosing Resistor Values for a Target

If the required feedback voltage is known, rearrange the equation to choose a resistor ratio:

R1 / R2 = Vout / Vf − 1

For instance, a target of 1.0 V at the feedback node when the output is 5.0 V requires R1/R2 = 5/1 − 1 = 4. A nominal pair such as 40 kΩ and 10 kΩ has that ratio, giving 1.0 V in the ideal unloaded calculation. This is an illustrative ratio, not a recommended component choice: the device datasheet may specify constraints on resistance, bias current, capacitance, or stability.

When selecting real values, consider more than the mathematical ratio:

• Resistor tolerance: Actual resistance varies from its nominal value, so the feedback voltage varies too. Check worst-case combinations if output accuracy matters.
• Power dissipation: The divider draws current. Its approximate current is Vout/(R1 + R2), and resistor power can be calculated from current and resistance. Confirm ratings against circuit conditions.
• Feedback-pin current: Input bias or leakage current can load a high-resistance divider and shift the junction voltage.
• Noise and power use: Lower resistance values generally make divider current larger; higher values reduce current but can make loading, noise, and parasitic effects more significant.
• Specified operating range: Use the controller’s data sheet and reference circuit for output limits, resistor ranges, filtering, and stability requirements.

What the Calculator Assumes

The calculation assumes a steady DC output, ideal resistors, and a feedback input that draws no current. It also assumes the divider is connected in the standard arrangement: R1 from output to the feedback node, R2 from that node to ground. Reversing the resistor positions changes the fraction. A load connected to the junction acts in parallel with R2 and changes the result; in that case, the simple formula is no longer exact unless the load is included as part of the lower-leg equivalent resistance.

The calculator accepts zero as a resistor input because zero is mathematically meaningful in some limiting cases, but a real feedback divider normally uses positive resistance values. If both resistors are zero, the formula divides by zero and has no defined result. Do not treat a numerical output as a substitute for checking whether the selected values meet the circuit’s electrical requirements.

Common Mistakes and Checks

• Swapping R1 and R2: R2 is the resistor from the feedback junction to ground in this formula. Putting the wrong value in that field produces the wrong fraction.
• Mixing units inconsistently: Enter both resistances in the same unit. For example, 10 kΩ and 10 kΩ give the same ratio as 10,000 Ω and 10,000 Ω.
• Confusing feedback voltage with output voltage: The calculator’s main result is the junction voltage, not the regulated output. The ratio explains how the two relate.
• Ignoring loading: Feedback-pin current, a measurement instrument, or another circuit connected to the junction can alter the voltage, especially with large resistor values.
• Rounding too early: Keep extra digits during intermediate calculations, then round the final value to the precision useful for the design. A displayed result does not imply that real components have that exact precision.

Frequently Asked Questions

What is the feedback voltage divider formula?
For R1 between output and feedback, and R2 between feedback and ground, use Vf = Vout × R2/(R1 + R2). The formula assumes the feedback node is not significantly loaded.

How do I calculate the output voltage from a feedback divider?
Rearrange the equation: Vout = Vf × (R1 + R2)/R2. This gives an ideal output for a known feedback voltage and resistor pair; the circuit’s reference voltage and operating limits must come from its documentation.

Does resistor order matter?
Yes. R1 is the upper resistor from output to the junction, while R2 is the lower resistor from the junction to ground. The equation uses the lower resistor in the numerator.

Can I use kilohms instead of ohms?
Yes. Use the same unit for both resistors. Since the calculation uses a ratio, entering both values in kilohms gives the same result as entering both in ohms.

Why does my measured feedback voltage differ from the calculated value?
Possible causes include resistor tolerance, feedback-input current, a load on the junction, wiring errors, output ripple, or measuring while the circuit is not in steady state. Compare the actual circuit with the device datasheet and account for those effects.

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in r1 | Upper resistor R1 | ohm | 10 | 0 | 1000000000000
in r2 | Lower resistor R2 | ohm | 10 | 0 | 1000000000000
in vo | Output voltage Vout | V | 5 | 0 | 1000000000
out vf | Feedback voltage Vf | V | vo * r2 / (r1 + r2) | 3
out ratio | Feedback ratio | - | r2 / (r1 + r2) | 4